Eukaryotic chromatin is composed of macromolecular complexes called nucleosomes. A nucleosome has 147 base pairs of DNA wrapped around a protein octamer having two subunits of each of histone protein H2A, H2B, H3, and H4. Histone proteins are subject to post-translational modifications which in turn affect chromatin structure and gene expression. One type of post-translational modification found on histones is methylation of lysine and arginine residues. Histone methylation plays a critical role in the regulation of gene expression in eukaryotes. Methylation affects chromatin structure and has been linked to both activation and repression of transcription (Zhang and Reinberg, Genes Dev. 15:2343-2360, 2001). Enzymes that catalyze attachment and removal of methyl groups from histones are implicated in gene silencing, embryonic development, cell proliferation, and other processes.
One class of histone methylases is characterized by the presence of a SET domain, comprising about 130 amino acids. EZH2 is an example of a human SET-domain containing methylase. EZH2 associates with EED (Embryonic Ectoderm Development) and SUZ12 (suppressor of zeste 12 homolog) to form a complex known as PRC2 (Polycomb Group Repressive Complex 2) having the ability to tri-methylate histone H3 at lysine 27 (Cao and Zhang, Mol. Cell 15:57-67, 2004). PRC2 complexes can also include RBAP46 and RBAP48 subunits. Another example is the related methylase EZH1.
The oncogenic activities of EZH2 have been shown by a number of studies in various different cancer types. ˜15-20% GCB-DLBCLs harbor a gain-of-function mutation in EZH2 (Y641 residue) and these cells are hypersensitive to EZH2 inhibition both in vitro and in vivo (McCabe et al, 2012; Bradley et al, 2014). In cell line experiments, over-expression of EZH2 induces cell invasion, growth in soft agar, and motility, while knockdown of EZH2 inhibits cell proliferation and cell invasion (Kleer et al., 2003, Proc. Nat. Acad. Sci. USA 100:11606-11611; Varambally et al., (2002), “The polycomb group protein EZH2 is involved in progression of prostate cancer,” Nature 419, 624-629). It has been shown that EZH2 represses the expression of several tumor suppressors, including E-cadherin, DAB2IP and RUNX3 among others. In xenograft models, EZH2 knockdown inhibits tumor growth and metastasis. It has been shown that down modulation of EZH2 in murine models blocks prostate cancer metastasis (Min et al., “An oncogene-tumor suppressor cascade drives metastatic prostate cancer by coordinately activating Ras and nuclear factor-kappaB,” Nat Med. 2010 March; 16(3):286-94). Recently, it was demonstrated that EZH2 is over-expressed in neuroendocrine tumors and inhibition of EZH2 in mouse tumors restore androgen dependence (Ku et al, Science, 355, 2017). EZH2 overexpression is associated with aggressiveness of certain cancers such as breast cancer (Kleer et al., Proc. Nat. Acad. Sci. USA 100:11606-11611, 2003). Recent studies also suggest that prostate cancer specific oncogenic fusion gene TMPRSS2-ERG induces repressive epigenetic programs via direct activation of EZH2 (Yu et al., “An Integrated Network of Androgen Receptor, Polycomb, and TMPRSS2-ERG Gene Fusions in Prostate Cancer Progression,” Cancer Cell. 2010 May 18; 17(5):443-454).
Given their role in the regulation of diverse biological processes, methyl modifying enzymes, in particular EZH2 and mutant forms thereof, are attractive targets for modulation.